Electric heating units and methods of making the same



Oct. 28, 1958 J. ANDREWS ELECTRIC HEATING UNITS AND METHODS OF MAKING THE SAME Original Filed Feb. 27. 1956 INVENTOR. Jo/m L. Andrews Afp/s assenti ELECTRIC liilEAliNG UNETS AND METHODS F MAKING THE SAME John L. Andrews, Chicago, lli., assigner to'General Electric Company, a corporation of New Yori:

Original application February 27, 1956, Serial No. 567,759. Divided and this application March 29, 1957, Serial No. 649,386

', Claims. (Cl. 2631-66) The present invention relates to electric heating units of the sheathed resistance conductor type and to methods of making the same. This application is a division of the copcnding application of John L. Andrews Serial No. 567,759, liled February 27, 1956.

It is a general object of the invention to provide an improved electric heating unit of the sheathed resistance conductor type and comprising a plurality of elongated helical resistance conductors arranged in multilar spaced-apart relation, a co-rresponding plurality of conductive terminals respectively electrically connected to the adjacent ends of the resistance conductors, an elongated metallic sheath enclosing the resistance conductors and the inner ends of the conductive terminals, with the outer ends of the conductive terminals projecting from the adjacent end of the sheath, and a dense mass of a -highly compacted refracto-ry material embedding the resistance conductors and the inner ends of the conductive terminals and retaining the resistance conductors and the conductive terminals vin proper spaced-apart relation with respect to each other and with respect to the sheath.

Another object of the invention is to provide a method of making an electric heating unit of the sheathed 'resistance conductor type of the character described; wherein the method involves improved steps that positively insure maintenance of proper spacing of the turns of the helical resistance conductors and of the conductive terminals therefor incident to the manufacture of the electric heating unit, and particularly during a cornpacting step involved in the method of manufacture thereof, wherein the dense mass of highly compacted refractory material is produced from a corresponding porous packing of such refractory material.

A further object of the invention is to provide a method of forming the terminals o-f an electric heating unit of the character described, that involves improved steps that positively insure maintenance of proper spacing of the terminals incident to compacting of the embedding refractory material in the enclosing metallic sheath, thereby to insure proper spacing of the terminals in the finished heating unit.

Further features of the invention pertain to the particular arrangement of the elements of the electric heating unit and of the steps of the method, whereby the above-outlined and additional operating features thereof are attained.

The invention, both as to its organization and method of operation, together with further objects and advantages thereof, will best be understood by reference to the following specication taken in connection with the accompanying drawing, in which:

Figure l is an enlarged fragmentary sectional view of an assembly that is employed in making an electric heating unit of the sheathed resistance conductor type in accordance with the method of the present invention;

Fig. 2 is an enlarged fragmentary sectional view, at

tes Patent O 2,858,401 Patented Oct. `25%, 158

two different scales, of the 'finished electric heating u'm't that has been produced from the assembly of Fig. l, the midportion of the heating unit of Fig. 2 being at a greatly enlarged scale for the purpose of illustrating diagrammatically the dilerent compositions of the refractory material embedding the resistance conductors in the enclosing sheath, and embodying the present invention; and

Fig. 3 is a perspective view of an electric -heater in the form yof a coil of a hotplate that has been produced 'from the finished electric heating unit of Fig. 2.

Referring now to Fig. 2 of the drawing, the heating unit 40 there illustrated, and embodying the features of the present invention and made in accordance wlth the method thereof, fundamentally comprises an `elongated tubular metallic sheath 41 that may be formed lof a suitable nickel-chromium-iron allo-y and having a substantially Acircular cross-section, and a pair of elongated helical resistance conductors or elements 42 and 43 that may be formed of a suitable nickel-chromium alloy and arranged in bifilar relation and located substany tially centrally within the sheath 41 and embedded! in 'a body of heat-conducting and electrical-insulating material of composite structure including a central core portion 44 formed of crystalline magnesium oxide and disposed within the turns of the resistance conductors '42 and 43, an intermediate layer or portion 45 of anhydrous amorphous magnesium oxide and disposed upon the core 44 and between the turns of the resistance conductors 42 and 43, and an outer layer or portion 46 of crystalline magnesium oxide and disposed upon the iresistance conductors 42 and 43 and upon the intermediate layer 45 and extending into contact with the sheath 41. The position and arrangement of the portions p44, 45 and 46 of the composite body of highly compacted refractory material disposed in the sheath 41 and embedding the resistance conductors 42 and 43, retaining the same in place and in proper spaced-apart relation with respect to each other and to the sheath 41, are shown diagramrnatically in the enlarged fragmentary midportion of Fig. 2.

Also, the unit 40 comprises a pair'of elongated conductive terminals 47 and 48 arranged in one-end of the sheath 41 and respectively electrically connected `at the inner ends thereof to the adjacent ends of the resistance conductors 42 and 43, and a pair of elongated conductive terminals 49a and 49h arranged in the other end ofthe sheath 41 and respectively electrically connected at the inner ends thereof to the adjacent ends of the resistance conductors 42 and 43. Thus the helical resistance c'onductor 42 extends between the terminals 47 and 4%@ the opposite ends thereof being respectively electrically connected thereto, as by welding; while the helicalresistance conductor 43 extends between the terminals 48 and 49h, the opposite ends thereof being respectively connected thereto, as by welding; and as previously noted, the elongated helical resistance conductors 42 and 43 are wound in bilar relation. Accordingly, it will be appreciated that the electric circuit of the resistance conductor 42 between the terminals 47 and 49a is entirely independent of the electric circuit of the resistance conductor 43 between the terminals 48 and 49]). Specical- 1y, at least two turns of the resistance conductor 42 at one end thereof are welded to two longitudinally spaced-v apart and outwardly directed ridges provided on the inner end of the terminal 47 and separated by an intervening notch in which one turn of the resistanceconductor 43 is disposed in spaced-apart relation withrespect to the inner end of the terminal 47. Similarly, at least two turns of the resistance conductor 43 at one end thereof are welded to two longitudinally spaced-apart and outwardly directed ridges provided on the inner end of the terminal 48 and separated by an intervening notch in which one turn of the resistance conductor 42 is disposed in spaced-apart relation with respect to the inner end of the terminal 48. Accordingly, it will be understood that while the inner ends of the terminals 47 and 48 extend within the adjacent ends of the biflar wound resistance conductors 42 and 43, the terminal 47 is electrically connected only to the resistance conductor 42 and the terminal 48 is electrically connected only to the resistance conductor 43.

The arrangement and relationship of the resistance conductors 42 and 43 and the respective terminals 49a and 49b at the other end of the heating unit 40 are the same as those described above in conjunction with the one end of the heating unit 40 and are not reiterated in the interest of brevity; whereby it will be understood that the terminal arrangement at the opposite ends of the heating unit 40 are identical.

Finally, -in the heating unit 40, glass insulating plugs 50 and 51 are respectively provided in the opposite ends of the sheath 41 and respectively seal the same against the entrance of moisture thereinto and respectively embed the terminal pairs 47-48 and 4961-4912.

In the manufacture of the heating unit 40 of Fig. 2, there is first produced the assembly illustrated in Fig. 1; and initially there is provided a lower subassembly including a pair of complementary fixtures 61 and 62 and an upper subassembly including a pair of complementary fixtures 71 and 72. The fixtures 61 and 62 are detachably secured together and provide, when assembled, a composite head and plug and are respectively provided with the two longitudinally extending and laterally spaced apart shanks that are ultimately fashioned into the respective terminals 47 and 48. Similarly, the fixtures 71 and 72 are detachably secured together and provide, when assembled, a composite head and are respectively provided with the two longitudinally extending and laterally spaced-apart Shanks that are ultimately fashioned into the respective terminals 49a and 49h. Accordingly, the i lower subassembly of the two fixtures 61 and 62 is substantially U-shaped including the junction or head and the two longitudinally extending Shanks 47 and 48 and likewise, the upper subassembly of the two fixtures 71 and 72 is substantially U-shaped including the junction or head and the two longitudinally extending shanks 49a and 49b.

In producing the assembly of Fig. l, the two elongated resistance conductors 42 and 43 and two elongated spacing members 52 and 53 are provided and then wound in multifilar relation upon an assoicated mandrel, not shown, the spacing members 52 and 53 being formed essentially of magnesium metal; whereby in the arrangement the sequence of the parts is as follows: a turn of the resistance conductor 42, a turn of the spacing member 52, a turn of the resistance conductor 43, and a turn of the spacing member 53. Accordingly, between each two adjacent turns of either one of the resistance conductors 42 or 43, there is disposed a turn of the other of the resistance conductors 43 or 42, as well as a turn of each of the spacing members 52 and 53. The composite helix is then removed from the mandrel and the opposite ends thereof are respectively assembled with respect to the lower and upper subassemblies. Specifically, the lower ends of the resistance conductors 42 and 43 are suitably secured by welding, or the like, to the respective shanks 47 and 48 of the lower assembly; and likewise, the upper end of the resistance conductors 42 and 43 are suitably secured by Welding, or the like, to the respective shanks 49a and 49h of the upper subassembly. In these welding operations, if employed, the fabricator is cautioned not to allow the spacing members 52 and 53 to become heated to an elevated temperature, as they are subject to ignition in air, being 4 formed of magnesium metal. Then this subassembly is arranged within the tubular sheath 41; and the extreme lower end of the sheath 41 is deformed into engagement with the head provided on the lower subassembly, as indicated at 41a, so as to close the lower end of the sheath 41.

At this time, the assembly is transferred to a loading machine of the character of that disclosed in U. S. Patent No. 2,316,659, granted on April 13, 1943 to John L. Andrews; whereby the head of the upper subassembly cooperates with the hook (indicated at 81) of the loading machine, so as to retain the composite multilar helix in depending position from the upper subassembly and substantially centrally Within the tubular sheath 41. The loading machine is then operated in a conventional manner; whereby a charge of finely divided crystalline mag nesium oxide is introduced into the upper end of the sheath 41 adjacent to the upper subassembly and into embedding relation with respect to the elements 42, 43, 52 and 53 of the composite multifilar helix, as well as the shanks 47 and 48 of the lower subassembly and the shanks 49a and 49h of the upper subassembly, thereby to produce the assembly, as shown in Fig. 1. In the loading machine, the lower subassembly serves as a stopper or plug to prevent the loss of the finely divided crystalline magnesium oxide from the sheath 41 as it is subjected to the usual tamping or hammering action in the loading machine. The action of the loading machine is such as to produce a firm, but porous, packing filling the central core surrounded by the elements 42, 43, 52 and 53 of the composite helix, as well as the space surrounding the parts 42, 43, 52, 53, 47, 48, 49a and 49b, and enclosed by the sheath 41, as illustrated in Fig. 1.

After filling of the sheath 41, the assembly is removed from the loading machine mentioned and the upper end of the sheath 41 is closed by an associated insulating plug and metal backing washer, not shown, the extreme upper end of the sheath 41 being bent over slightly to retain the insulating plug and metal backing washer in place.

At this time the assembly thus produced is transferred to a rolling machine of the character of that disclosed in U. S. Patent No. 2,677,172, granted on May 4, 1954 to Sterling A. Oakley; and the rolling machine is operated in order to subject the assembly to a preliminary compacting step, the assembly being moved upwardly while supported in an upright position through the several cold rolling passes of the rolling machine. Specifically, in the preliminary rolling operation, the diameter of the sheath 41 is modestly reduced so as modestly to reduce the cross-sectional area thereof and of the charge of refractory material and so as to crush the plug, not shown, arranged in the upper end of the sheath 41, With the result that there is obtained the corresponding modest compacting of the refractory material in rthe sheath 41 and into supporting relation with the elements 42, 43, 52, 53, 47, 48, 49a and 49h. At this time, after the prelimi nary rolling step, the charge of crystalline magnesium oxide, still porous, and the interior of the sheath 41 are arranged in communication with the atmosphere by virtue of the crushing of the plug, not shown, in the upper end of the sheath 41.

Following the preliminary rolling step, the assembly, having substantially the appearance as illustrated in Fig. l, is transferred to an autoclave of conventional constrnction including a heating chamber, an associated electric heater, a connecting vacuum pump. a connecting oxygen tank provided with a pressure regulator, pressure gauges, and suitable valves and fittings, all of a known character. In the autoclave, the assembly is subjected to heat-treatment in order to effect oxidation, in any suitable manner, of the spacing members 52 and 53, so that they are converted from magnesium metal to anhydrous amorphous magnesium oxide; and preferably, the steps that are carried out in the autoclave mentioned are in accordance with the process disclosed in the copending application of Emmett W. Barnes, Serial No. 567,849, led February 27, v1956.

In accordance with the Barnes process, the heating chamber of the yautoclave is closed after the assembly is loaded thereinto; and actually the heating chamber is arranged to receive a substantial group or number of the assemblies, as a matter of production facility. After closing and sealing of the Vheating chamber, it is evacuated and the vacuum is held for about l5 minutes, so as toV remove air from the porous packing of refractory material enclosed in the sheath 41. Thereafter, gaseous oxygen is introducedk into the heating chamber under gauge pressure of about 70 p. s. i., and the assembly is soaked thereink for about l5 minutes. Again the heating chamber is evacuated and the vacuum is held for about l5 minutes, so as to remove residual air from the porous packing of refractory material enclosed in the sheath 41. Thereafter, gaseous oxygen is again introduced into the heating chamber under gaugepressure of about 70 p. s. i.; and this pressure is maintained therein in order again to permeate the packing of refractory materialand to contact the magnesium metal spacing members 52 and`53. The temperature of the heating chamber is then elevated to a reaction temperature disposed below the ignition temperature of magnesium metal at the gauge pressure of about 70 p. s. i., which elevated reaction temperature is maintained for a suiiicient time interval completely to react the magnesium metal spacing members 52 and 53, so as completely to convert the same to anhydrous amorphous magnesium oxide; whereby the mass of amorphous magnesium oxide comprising the layer 45 is produced in the assembly, as indicated in Fig. 2. l

It has been discovered that under the reaction conditions speciiied, the ignition temperature of magnesium metal is about 1085 F., whereby the reaction temperature is maintained in the heating chamber atabout l075 F., this reaction temperature'being safely below-the igni- `tion temperature of magnesium metal, yet suiciently close thereto to insure a high rate of reaction. It is estimated that in the manufacture of the heating unit 40, the required time interval of the reaction is about 8 hours; however, the assembly is retained in the autoclave under the reaction conditions set forth for a time interval of about 16 hours, so as positively to insure the complete conversion of the spacing members 52 and 53 from magnesium metal to magnesium oxide. Specifically, this time interval is not critical, provided it is sutiiciently long to insure the complete reaction of all of the magnesium metal and the complete conversion thereof to amorphous magnesium oxide; i. e., the maintenance of the reaction conditions in the heating chamber of the autoclave for some time interval following the complete conversion of the magnesium metal to amorphous magnesium oxide is in nowise deleterious, since the reaction is automatically terminated when the conversion is completed.

After the complete reaction of the magnesium metal spacing members A52 and 53 has been achieved in the autoclave, the heating chamber thereof is opened to the atmosphere and allowed to cool; whereupon the assembly is removed from the heating chamber and again transferred to the rolling machine; whereupon it is subjected to a final compacting step, similar to 1the preliminary compacting step previously described, but substantially more severe. In the final rolling operation, the diameter of the sheath y41 is substantially reduced so -as substantially to reduce the cross-sectional area of the refractory material embedding the helical resistance conudructors 42 Vand 43, and retaining the same in spaced-apart relation with respect to each other and vwith respect to 'the sheath 41. The dense 'mass of compacted refractory material also positions and retains in place the terminals 47, 48, 49a and 49b. p v

Following the iinal rolling step in the rolling Vmachine, the -lower end of the sheath 41 and the lower subassembly are cutoff or severed to produce the two separate and distinct terminals 47 and 48; and likewise, the upper end of the sheath 41 and the upper subassembly are `cut oi to produce the two separate and distinct terminals 49a and 49h. Ultimately, the terminals 47 and 4S and the terminals 49a and 49h are respectively bent outwardly with respect'to each other, and then the glass lplugs 50 and 51 are cast into the opposite ends ofthe tubular sheath 41 so as to seal the same and to produce the finished heating unit 40, as illustrated in Fig. 2.

In a modification of the method of making the heating unit 40, the assembly is subjected to heat-treatment inthe autoclave in -the manner described, only throughout a lrelatively short time interval so as to effect only lthe conversion of the outer skins of the magnesium helices 52y and 53 into amorphous magnesium oxide, leaving the interior cores thereof as the original magnesium metal. For example, the outer skins of ythe helices 52 and 53 may be converted into magnesium oxide, so that the conversion of the helices 52 and 53 is Vabout 10% complete, by treatment in the autoclave, in the manner described, throughout a time interval of only about two hours.

After manufacture of the heating unit 40, it is ordinarily subjected to the usual electrical tests in order to determine the characteristics thereof (insulation resistance, break-down voltage, heat distribution, etc.) affecting the performance and expected life thereof in service.

-As a constructional example of the manufacture of the heating unit 40, the tubular sheath 41 may have an initial diameter of O.3 12"; and in the preliminary rolling step, the diameter thereof may be reduced to 0299"; and thereafter in the final rolling step, -the diameter thereof may be reduced to 0.270. In the heating unit 40: the resistance conductor 42 may be formed of #30A (B. & S.) gauge wire having a diameter of'about 0.0ll0; the resistance conductor 43 may be formed of #31A (B. & S.) gauge wire having a diameter of about 0.0089; and each of the spacing members 52 and 53 may be formed of magnesium wire having a diameter of about 0.0140. In the assembly of Fig. l, the spacing between each resistance conductor 42 and 43 and the associated spacing .members 52 and 53 may be about 0.0010, thereby accommodating the passage of the finely divided crystalline magnesium oxide therethrough into the interior of the composite multiilar helix incident to operation of the loading machine. Also, in the assembly of Fig. l, the outside diameter of the turns of the composite multililar helix may be about 0.110.

Of course, the number of turns per inch longitudinally of the composite multiiilar helix of each of the elements 42, 43, 52 and 53 is dependent fundamentally upon the gauges of the resistance conductors 42 and 53 that,

in turn, is dependent upon the desired wattage rating ofthe finished heating unit 40. However, as a practical matter, it has been discovered that from 10 to 2l turns per inch longitudinally of the composite multifilar helix `is feasible, when both of the resistance conductors 42 and 43l Vare formed respectively of relatively coarse and relatively fine resistance wire.

After the manufacture of the heating unit 40, it is incorporated in an appliance, or the like, such,.for example, as an electric hotplate, as illustrated in Fig. 3. Specifically, a heating element or coil 30 of a hotplate is illustrated in Fig. 3; and the heating element 30 is formed from the heating unit 4l) by appropriate bending thereof into the required configuration followed by flattening of the upper surface of the sheathv41, as indicated at 41C. This flattening of the upper surface or top ofthe sheath y41, as indicated at 41C, of the heating element 30 not only accommodates the ready support of a cooking vessel to be heated, but it also effects further tightening or compacting of the mass of refractory material enclosed in the sheath 41, as well as the elimination of any cracks or fissures in the refractory material that might be produced therein incident to the bending of the heating unit 40 into the desired configuration of the heating element 30.

In the manufacture of the heating unit 40, it will be understood that during loading and tamping in place of the charge of finely divided crystalline magnesium oxide into the sheath 41 in the loading machine, that the pair of spacing members 52 and 53 formed of magnesium rnetal serve the function of retaining in place the pair of resistance conductors 42 and 43; which arrangement is very advantageous in view of the close longitudinal spacing between adjacent turns of the resistance conductors 42 and 43. Thereafter when the spacing members 52 and 53 formed of magnesium metal are oxidized in the autoclave into anhydrous amorphous magnesium oxide, there is a corresponding expansion of the volume occupied thereby within the enclosing sheath 41, since there is an expansion of about 200% when a given mass of magnesium metal is converted into a corresponding mass of anhydrous amorphous magnesium oxide by oxidation. This action of course produces further compacting of the resulting mass of refractory material in the sheath 41 of the heating unit 40 further eliminating voids therein and further contributing toward the production of a dense mass of refractory material in the finished heating unit 4t) embedding the resistance conductors 42 and 43 and retaining the same in place. This feature is very advantageous, as it will be understood that the composite mass of refractory material mentioned not only serves the mechanical functions described with respect to holding the resistance conductors 42 and 43 in place in in-` sulated condition with respect to the surrounding sheath 41, Vbut it also serves the function of transmitting the heat produced in the resistance conductors 42 and 43 to the sheath 41 for the useful heating purpose. It is emphasized that the last-mentioned function is very important as it not only contributes to efficiency of the finished electric heating unit 40, but it also prevents excessive temperatures of the resistance conductors 42 and 43, thereby materially contributing to desirable long life of the heating unit 40.

Again referring to Fig. 2, it is pointed out that the peculiar arrangement of the layer 4S of amorphous magnesium oxide between the turns of the resistance conductors 42 and 43 and in an intermediate position with respect to the central core 44 of crystalline magnesium .oxide and the outer layer 46 of crystalline magnesium oxide is most advantageous due to the circumstances that amorphous magnesium oxide has both a higher coefficient of electrical resistance and a lower coefficient of heat conductivity than does crystalline magnesium oxide. Thus: the amorphous magnesium oxide of the layer 45 is a better electrical insulator between adjacent turns of the resistance conductors 42 and 43 than are the layers 44 and 46 of crystalline magnesium oxide; and moreover, the amorphous magnesium oxide of the layer 45 is a poorer heat conductor between adjacent turns of the resistance conductors 42 and 43 than are the layers 44 and 46 of crystalline magnesium oxide. Accordingly, the layer 4S of amorphous magnesium oxide is substantially ideally suited to utilization between the turns of the resistance conductors 42 and 43, since it has both a high electrical insulating characteristic and a low heat conducting characteristic, thereby minimizing undesirable electrical and heating effects between the resistance conductors 42 and 43, without in any way impairing the eiiiciency of the heating unit 4t).

In conjunction with the operation of the autoclave, the processor is cautioned that the reaction temperature maintained in the heating chamber thereof must not be permitted to rise to the ignition temperature of magnesium metal, since it will be apparent that the ignition of the magnesium metal will bring about the production of an exceedingly high temperature, with the consequent melting of the adjacent resistance conductors, or even the enclosing sheath. of the heating unit undergoing the heat treatment. Fortunately the ignition temperature of magnesium metal in gaseous oxygen at a gauge pressure of about 70 p. s. i. is well-defined at 1085 F., whereby the reaction temperature of 1075 F. is entirely safe for this step. in this connection, it is noted that the ignition temperature of the magnesium metal is related to the pressure of the atmosphere of gaseous oxygen, the ignition temperature increasing with increasing pressures of the gaseous oxygen atmosphere. Thus, it will be appreciated that the reaction temperature-gauge pressure relationship mentioned is capable of appropriate variation dependent upon the factors noted; however, from a practical standpoint, the relatively low gauge pressure of about 70 p. s. i. and the readily controllable reaction temperature of .about 1075 F. are recommended for commercial production of the heating units in accordance with the present method.

In the foregoing explanation of the present method, the members 52 and 53 of the assembly of Fig. 1 were described as being formed essentially of magnesium metal; however, a modification is contemplated, wherein these members are formed essentially of beryllium, magnesium, aluminum or titanium, or alloys thereof, as it will be understood that the elements named comprise a welldefned group of metals that may be readily converted from the metallic form into the corresponding metal oxide by oxidation with gaseous oxygen under gauge pressure and at an elevated temperature in the autoclave in a manner substantially identical to that described, and wherein each of the corresponding metal oxides constitutes a refractory material having good electrical-insulating and at least moderately good heat-conducting properties. In each case, the operating temperature of the heating chamber of the autoclave is established somewhat below the ignition temperature of the corresponding metal or alloy, so as to prevent ignition of the metal members in the atmosphere of oxygen gas under gauge pressure at the elevated temperature.

In view of the foregoing, it is apparent that there has been provided both an electric heating unit of improved `construction and arrangement, as well as an improved method of making the same, that may be readily carried out upon a mass production basis for commercial purnoses.

While there has been described what is at present considered to be the preferred embodiment of the invention, it will be understood that various modifications may be made therein, and it is intended to cover inthe appended claims all such modifications as fall within the true spirit and scope of the invention.

What is claimed is:

1. The method of making an electric heating unit of the sheathed resistance conductor type, which comprises: providing a pair of elongated resistance conductors, winding said resistance conductors into helical form in bifilar relation with a turn of one of said resistance conductors arranged between each two adjacent turns of the other of said resistance conductors, providing a substantially U-shaped conductive member including a junction `and a pair of elongated legs extending therefrom and disposed in spaced relation with respect to each other, respectively connecting the ends of said legs to the adjacent ends of said resistance conductors, providing an elongated tubular metallic sheath, enclosing said winding of said resistance conductors in said sheath and in spaced relation therewith and arranging said junction exteriorly of the adjacent one end of said sheath with the ends of said legs projecting thereinto and in spaced relation therewith, loading a charge of refractory material into said sheath in embedding relation with said resistance conductors and the ends of said legs and so as toy retain the same in place in spaced relation with respect to said sheath, working the assembly of said elements named to reduce the cross-sectional area of said sheath in order to compact said charge of refractory material into a dense mass still embedding and retaining in place said resistance conductors and the ends of said legs, and then severing said junction from the'portions of said legs projecting from the adjacent end of said sheath to define a pair of separate and distinct conductive terminals for the adjacent ends of said resistance conductors.

2. The method of making an electric heating unit of the sheathed resistance conductor type, which comprises: providing a pair of elongated resistance conductors, winding said resistance conductors into helical form in biflar relation with a turn of one of said resistance conductors arranged between each two adjacent turns of the other of said resistance conductors, providing a substantially U-shaped conductive member including a junction and a pair of elongated legs extending therefrom and disposed in spaced relation withrespect toy each other, respectively connecting the ends of said legs to the adjacent ends of said resistance conductors, providing an elongated tubular metallic sheath, enclosing said winding of said resistance conductors in said sheath and in spaced relation therewith and arranging said junction exteriorly of the adjacent one end of said sheath with the ends of said legs projecting thereinto and in spaced relation therewith, arranging the assembly of said elements named in an upstanding position with said one end of said sheath at the top thereof and with said resistance conductors depending from said legs into said sheath, introducing a charge of finely divided refractory material into said one end of said sheath in embedding relation with said resistance conductors and the ends of said legs so as to retain the same in place in spaced relation with respect to said sheath, working said assembly to reduce the cross-sectional area of said sheath in order to compact said charge of refractory material into a dense mass still embedding and retaining in place said resistance conductors and the ends of said legs, and then severing said junction from the portions of said legs projecting from said one end of said sheath to define a pair of separate and distinct conductive terminals for the adjacent ends of said resistance conductors.

3. The method of making an electric heating unit of the sheathed resistance conductor type, which comprises: providing a pair of elongated resistance conductors, winding said resistance conductors into helical form in biiilar relation with a turn of one of said resistance conductors arranged between each two adjacent turns of the other of said resistance conductors, providing a substantially U- shaped conductive member including a junction and a pair of elongated legs extending therefrom and disposed in spaced relation with respect to each other, respectively connecting the ends of said legs to the adjacent ends of said resistance conductors, providing an elongated tubular metallic sheath, enclosing said winding of said resistance conductors in said sheath and in spaced relation therewith and arranging asid junction as a plug in the adjacent one end of said sheath with the ends of said legs projecting therefrom inwardly into said sheath and in spaced relation therewith, loading a charge of refractory material into said sheath in embedding relation with said resistance conductors and the ends of said legs Iand so as to retain the same in place in spaced relation with respect to said sheath, working the assembly of said elements named to reduce the cross-sectional area of said vsheath in order to compact said charge of refractory material into a dense mass still embedding land retaining in place said resistance conductors and the ends of said legs, severing said one end of said sheath to expose and to position said junction exteriorly thereof with the ends of said legs projecting into said one of said sheath,

and then severing said junction from the projecting portions of said legs to define a, pair of separate and distinct conductive terminals for the adjacentiends of said resistance conductors.

4. The method of making an electric heating unit of the sheathed resistance conductor type, whichcomprises: providing a pair of elongated resistance conductors, winding said resistance conductors into helical form in bilar relation with a turn of one of said resistance conductors arranged between each two adjacent turns of the other of said resistance conductors, providing a substantially U-shaped conductive member including a junction and a pair of elongated legs extending therefrom and disposed in spaced relation with respect to each other, respectively connecting the endsy of said legs to the adjacent ends of said resistance conductors, providing an elongated tubular metallic sheath, enclosing said winding of said resistance conductors in said sheath and in spaced relation therewith and arranging said junction as a plug in the adjacent one end of said sheath with the ends of said legs projecting therefrom inwardly into said sheath and in spaced relation therewith, arranging the assembly of said elements named in an upstanding position with said one end of said sheath at the bottom thereof, introducing a charge of nely divided refractory material into the other end of said sheath in embedding relation with said resistance conductors and the ends of said legs so as to retain the samein place in spaced relation with respect to said sheath, working the assembly of said elements named to reduce the cross-sectional area of said sheath in order to compact said charge of refractory material into a dense mass still embedding and retaining in place said resistance conductors and the ends of said legs, severing said one end of said sheath to expose and to position said junction exteriorly thereof with the ends of said legs projecting into said one end of said sheath, and then severing said junction from the projecting portions of said legs to define a pair of separate and distinct conductive terminals for the adjacent ends of said resistance conductors.

5. An electric heating unit comprising a pair of elongated helical resistance conductors arranged in bilar relation with a turn of one of said resistance conductors arranged between each two adjacent turns of the other of said resistance conductors and with the turns of said resistance conductors in spaced relation, an elongated tubular metallic sheath enclosing said resistance conductors and spaced therefrom, a pair of elongated terminals arranged in one end of said sheath and projecting therefrom to the exterior, said terminals being spaced mutually from each other and from said sheath, the inner end of one of said terminals projecting into the adjacent ends of both of said resistance conductors and extending into engagement with a number of the surrounding turns of one of said resistance conductors and spaced from the surrounding turns of the other of said resistance conductors, the inner end of the other of said terminals projecting into the adjacent ends of both of said resistance conductors and extending into engagement with a number of the surrounding turns of said other resistance conductor and spaced from the surrounding turns of said one resistance conductor, and a dense mass of compacted refractory material arranged in said sheath and embedding said resistance conductors and the inner ends of said terminals and retaining th'e same in place.

6. The electric heating unit set forth in claim 5, wherein the inner end of each one of said terminals is provided with a number of outwardly directed ridges respectively engaging the individual turns of the corresponding one of said resistance conductors and at least one intervening notch receiving but spaced from an individual turn of the other of said resistance conductors.

7. An electric heating unit comprising a pair of elongated helical resistance conductors arranged in bilar relation with a turn of one of said resistance conductors arranged between each two adjacent turns of the other of said resistance conductors and with the turns of said resistance conductors in spaced relation, an elongated tubular metallic sheath enclosing said resistance conductors and spaced therefrom, a pair of elongated terminals arranged in one end of said sheath and projecting therefrom to the exterior, said terminals being spaced mutually from each other and from said sheath, the inner end of one of said terminals being electrically connected to the adjacent end of one of said resistance conductors and spaced from the adjacent end of the other of said resistance conductors, the inner end of the other of said terminals being electrically connected to the adjacent end of said other resistance conductor and spaced from the adjacent end of said one resistance conductor, and a dense mass of compacted refractory material arranged in said sheath and embedding said resistance conductors and the inner ends of said terminals, said refractory material retaining said resistance conductors and said sheath in mutually spacedapart relation and also retaining said terminals and said sheath in mutually spaced-apart relation.

8. An electric heating unit comprising a pair of elongated helical resistance conductors arranged in bifilar relation with a turn of one of said resistance conductors arranged between each two adjacent turns of the other of said resistance conductors and with the turns of said resistance conductors in spaced relation, an elongated tubular metallic sheath enclosing said resistance conductors and spaced therefrom, a pair of elongated terminals arranged in one end of said sheath and projecting therefrom to the exterior, said terminals being spaced mutually from each other and from said sheath, a plurality of sections of the end of one of said resistance conductors being respectively arranged in a corresponding plurality of recesses provided in the surface of the inner end of one of said terminals and welded in place to provide an electrical connection therebetween, a plurality of sections of the end of the other of said resistance conductors being respectively arranged in a corresponding plurality of recesses provided in the surface of the inner end of the other of said terminals and welded in place to provide an electrical connection therebetween, the inner end of said one terminal being spaced from the adjacent end of said other resistance conductor and the inner end of said other terminal being spaced om the adjacent end of said one resistance conductor, and a dense mass of compacted refractory material arranged in said sheath and embedding said resistance conductors and the inner ends of said terminals, said refractory material retaining said resistance conductors and said sheath in mutually spacedapart relation and also retaining said terminals and said sheath in mutually spaced-apart relation.

References Cited in the ile of this patent UNITED STATES PATENTS 1,684,184 King Sept. 11, 1928 1,738,026 Wennagel et al Dec. 3, 1929 1,751,416 Musgrove Mar. 18, 1930 2,284,862 Ginder Ian. 2, 1942 2,494,333 Daly Ian. l0, 1950 FOREIGN PATENTS 46,408 Norway Apr. 29, 1929 UNITED STATES PATENT OFFICE CERTIFICATE 0F CORRECTION Patent Nm 229858,4@1 y October 289 1958 John Lo Andrews It is herebyT certified that error appears in the printed specification of the above numbered patent requiring correction and that the said-Letters Patent should read as corrected below Column 9, line 6ly for "esd" reed me Seiden; line '759 after "oneH insert m end ma Signed and sealed the 27th day of, January 1959 SEAL) ttest:

KARL EL, AXTITNE ROBERT C, WATSON Attesting Officer Commissioner of Patents UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent Nm 298584O1 Ootober 289 i958 Tom In Andrews It is hereby certified that error appears in the printed specification of the above numbered patent requiring correction and that the saidLetters Patent should read as corrected below.,

@Olm 99 lin@ 6l for "ESTE" read :e Sammy line '75y after "one" insert m end mm,

Signed and sealed thie 27th dey of January 19590 SEAL) ttest:

KARL Ho AXLINE Attesting Officer ROBERT C. WATSON Commissioner of Patents 

